If you've still got those disposable eclipse-watching glasses lying around from Sunday's partial solar eclipse -- don't throw them away.

The glasses can be used in the coming weeks to gaze at the sun again, this time to witness a far rarer celestial event. On June 5, Venus will journey across the surface of the sun, blocking a tiny circle of light as it goes.

There have been only six transits of Venus, as they're called, since telescopes were invented, and they occur in a patterns of eight years followed by 121.5 years followed by eight years, again, and then 105.5 years. Since Venus crossed the sun just eight years ago, in 2004, the next transit of Venus won't occur until Dec. 11, 2117.

Douglas Duncan, director of the University of Colorado's Fiske Planetarium, said people can use their eclipse-watching glasses to try to discern the black dot that is Venus as it crosses the sun. And for those who may already have thrown out their glasses -- or didn't buy any before they sold out -- more glasses were expected to be made available for purchase at McGuckin Hardware or Fiske Planetarium on Friday.

But even with the glasses, Venus may be hard to spot.

"Venus (appears) a whole lot smaller than the moon," Duncan said. "If you have good eyes, you can certainly try, and it's fun to try."

But if you need a little help, Fiske will have telescopes that have been fitted with the proper filters set up between 4 and 8 p.m. June 5 for the public to peek through. And Duncan and his staff will man a booth this weekend at the Boulder Creek Festival to show novice sun-gazers how to use a standard pair of binoculars to project an enlarged image of the sun onto another surface. Instructions also are available on Duncan's website, eclipse-watch.com.

The upcoming transit may delight the cosmically curious, but its scientific value is far less significant than it once was.

Hundreds of years ago, the transit of Venus across the sun helped astronomers, for the first time, get a handle on the distance between the Earth and the sun, and therefore, the scale of the whole solar system.

"Once upon a time, it was really important," Duncan said. "Before we knew the size of the solar system, this was a way of figuring out how far apart things were."

Astronomer Edmond Halley in the 1600s became the first to figure out that having multiple people observe the transit of Venus from far-flung locations across the globe could help crack the mystery of the universe's scale. But by the time Halley -- the namesake of Halley's Comet -- figured out the math, the next Venus transit was more than half a century away.

Halley died before it happened, but he left detailed instructions for future generations and wished them luck, especially over the tyranny of cloudy skies.

"I earnestly wish them all imaginable success," he wrote. "In the first place, that they may not, by the unseasonable obscurity of a cloudy sky, be deprived of this most desirable sight, and then that, having ascertained with more exactness the magnitudes of the planetary orbits, it may rebound to their immortal fame and glory."

The nut of Halley's idea is simple geometry, Duncan said.

"If you're driving down the road, and you're changing your position, the closer something is to you the more it appears to move," Duncan said.

In the same way, astronomers positioned across the globe could compare the position of Venus at different times as it moved across the sun to surmise the planet's distance from Earth: the greater the difference, the closer the distance.

And because astronomers already had worked out the relative position of the planets to each other, knowing the distance to one planet -- in this case, Venus -- allowed them to figure out the other distances as well.

"All they needed to do was measure the mileage between two things and then it would establish the scale of the whole solar system," Duncan said.

Halley's theory was put to test during the transits of Venus in 1761 and 1769. And while the observers -- some of whom had gone on great expeditions to arrive in Siberia, Africa and other remote locations -- had some problems measuring the transit due to the atmospheric interference, it still helped astronomers get their first estimate of cosmic distances.

The transit of Venus on June 5 will start around 4 p.m. and last through sunset in Colorado.

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